Apparatus and Method for Finger Activity on a Fingerprint Sensor

ABSTRACT

An apparatus and method for detecting the presence of a finger on a fingerprint sensor is disclosed in one embodiment of the invention as including transmitting a probing signal, comprising a series of probing pulses, to a fingerprint sensing area. A response signal, comprising a series of response pulses, is received from the fingerprint sensing area in response to the probing signal. An upper reference signal is generated and finger activity is detected on the fingerprint sensing area by monitoring whether the peaks of the response pulses exceed the reference signal.

CROSS-REFERENCE

This application is a continuation application of Ser. No. 12/354,722,filed Jan. 15, 2009, which is incorporated herein by reference in itsentirety and to which application we claim priority under 35 USC §120.

BACKGROUND

This invention relates to fingerprint sensors and more particularly toapparatus and methods for managing power consumption in fingerprintsensing circuits and also to apparatus and method for detecting fingeractivity with fingerprint sensing circuits.

Power management is increasingly important in today's mobile electronicdevices as greater reliance is placed on batteries and other mobileenergy sources. This is true for devices such as portable computers,personal data assistants (PDAs), cell phones, gaming devices, navigationdevices, information appliances, and the like. Furthermore, with theconvergence of computing, communication, entertainment, and otherapplications in mobile electronic devices, power demands continue toincrease at a rapid pace, with batteries struggling to keep pace. At thesame time, even where additional features and capability are provided inmodern electronic devices, consumers still desire elegant, compactdevices that are small enough to be slipped into a pocket or handbag.

While power management continues to increase in importance, accesscontrol is also becoming increasingly important as it relates to modernelectronic devices. Access control generally refers to methods andtechniques for restricting the ability of a user or program to access asystem's resources. Access control is gaining importance at least partlybecause users are storing increasing amounts of private, sensitive, orconfidential information on mobile electronic devices. The electronicdevices themselves are also valuable. Thus, restricting access to thesedevices may provide an effective deterrent to theft or misappropriationby reducing the value of the devices for would-be thieves or resellers.

Although reusable passwords are probably the most common technique forauthenticating and identifying a user of a device, various othertechniques are also being developed to counter the various ways thatreusable passwords may be compromised. For example, fingerprint sensorsprovide one potential method for identifying and authenticating a user.Fingerprints, like various other biometric characteristics, are based onan unalterable personal characteristic and thus are believed to morereliable to identify a user. Nevertheless, like other features,fingerprint and other biometric sensors typically require additionalhardware and software for implementation in electronic devices. Thishardware and software adds to the already large power demands beingplaced on these devices.

In view of the foregoing, what are needed are apparatus and methods forefficiently managing and conserving power in fingerprint sensingcircuits. For example, apparatus and methods are needed to significantlyreduce power consumed by fingerprint sensing circuits when the circuitsare idle or waiting for a user to apply a fingerprint. Further neededare methods and techniques to enable fingerprint sensors to quickly“wake up” when finger or non-finger related activity is detected by thecircuit. Further needed are apparatus and methods for determiningwhether a finger is or is not present on a fingerprint sensor whendetection begins.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific examplesillustrated in the appended drawings. Understanding that these drawingsdepict only typical examples of the invention and are not therefore tobe considered limiting of its scope, the invention will be described andexplained with additional specificity and detail through the use of theaccompanying drawings, in which:

FIG. 1 is a high level block diagram of one embodiment of a fingerprintsensing area associated with a fingerprint sensing circuit in accordancewith the invention;

FIG. 2 is a high level block diagram showing various components that maybe associated with the flow of data through the fingerprint sensingcircuit;

FIG. 3 is a high level block diagram of various components and powerdomains that may be associated with an integrated fingerprint sensingcircuit in accordance with the invention;

FIG. 4 is a state diagram showing various different power modes, andevents for transitioning between the power modes, that may be associatedwith a fingerprint sensing circuit in accordance with the invention;

FIG. 5 is a high level block diagram of one embodiment of a“wake-on-event” module, for use in a fingerprint sensing circuit inaccordance with the invention;

FIG. 6 is a high level block diagram of one embodiment of a detector foruse in a “wake-on-event” module in accordance with the invention;

FIG. 7 is a timing diagram showing the relationship between varioussignals in a fingerprint sensing circuit in accordance with theinvention;

FIG. 8 is another timing diagram showing the relationship betweenvarious signals in a fingerprint sensing circuit in accordance with theinvention;

FIG. 9 is a diagram showing one method for timing the probing pulsestransmitted to the fingerprint sensing area; and

FIG. 10 is a diagram showing another method for timing the probingpulses transmitted to the fingerprint sensing area.

DETAILED DESCRIPTION

The invention has been developed in response to the present state of theart, and in particular, in response to the problems and needs in the artthat have not yet been fully solved by currently available fingerprintsensors. Accordingly, the invention has been developed to provide novelapparatus and methods for managing power consumption in fingerprintsensing circuits. The invention has been further developed to providenovel apparatus and methods for detecting finger activity withfingerprint sensing circuits. The features and advantages of theinvention will become more fully apparent from the following descriptionand appended claims and their equivalents, and also any subsequentclaims or amendments presented, or may be learned by practice of theinvention as set forth hereinafter.

Consistent with the foregoing, a method for detecting the presence of afinger on a fingerprint sensor is disclosed in one embodiment of theinvention as including transmitting a probing signal, comprising aseries of probing pulses, to a fingerprint sensing area. A responsesignal, comprising a series of response pulses, is received from thefingerprint sensing area in response to the probing signal. An upperreference signal is generated and finger activity is detected on thefingerprint sensing area by monitoring whether the peaks of the responsepulses exceed the reference signal. In selected embodiments, the methodincludes counting the number of times the peaks exceed the upperreference signal. Accordingly, finger activity may be detected on thefingerprint sensing area based on the number of times the peaks exceedthe upper reference signal.

Similarly, in selected embodiments, the method may also includeestablishing a lower reference signal and monitoring whether the peaksof the response pulses drop below the lower reference signal. Removal ofa finger from the fingerprint sensing area may be detected by monitoringwhether the peaks of the response pulses drop below the lower referencesignal. In selected embodiments, the method includes counting the numberof times the peaks drop below the lower reference signal. Accordingly,removal of a finger from the fingerprint sensing area may be detectedbased on the number of times the peaks drop below the lower referencesignal.

In another embodiment of the invention, an apparatus for detecting thepresence of a finger on a fingerprint sensor includes a transmitterelement to emit a probing signal comprising a series of probing pulsesat a fingerprint sensing area. A receiving element receives a responsesignal comprising a series of response pulses from the fingerprintsensing area in response to the probing signal. A signal generator isprovided to generate an upper reference signal. A detector monitorswhether the peaks of the response pulses exceed the upper referencesignal. Similarly, detection logic may determine whether a finger hasbeen placed on the fingerprint sensing area based on whether the peaksexceed the upper reference signal.

In selected embodiments, the apparatus further includes a counter tocount the number of times the peaks exceed the upper reference signal.Similarly, the detection logic may detect finger activity based on thenumber of times the peaks exceed the upper reference signal.

In selected embodiments, the apparatus may also include a second signalgenerator to generate a lower reference signal. The detector may monitorwhether the peaks drop below the lower reference signal. Detection logicmay determine whether a finger has been removed from the fingerprintsensing area based on whether the peaks drop below the lower referencesignal. In selected embodiments, a counter may count the number of timesthe peaks drop below the lower reference signal. Similarly, thedetection logic may determine whether a finger has been removed from thefingerprint sensing area based on the number of times the peaks dropbelow the lower reference signal.

In yet another embodiment of the invention, an apparatus for detectingthe presence of a finger on a fingerprint sensor includes means fortransmitting a probing signal comprising a series of probing pulses to afingerprint sensing area. The apparatus further includes means forreceiving a response signal comprising a series of response pulses fromthe fingerprint sensing area in response to the probing signal. Theapparatus further includes means for establishing an upper referencesignal and means for monitoring whether the peaks of the response pulsesexceed the upper reference signal.

Similarly, in selected embodiments, the apparatus may also include meansfor establishing a lower reference signal and monitoring whether thepeaks of the response pulses drop below the lower reference signal.Removal of a finger from the fingerprint sensing area may be detected bymonitoring whether the peaks of the response pulses drop below the lowerreference signal. In selected embodiments, the apparatus may furtherinclude means for counting the number of times the peaks drop below thelower reference signal. Accordingly, removal of a finger from thefingerprint sensing area may be detected based on the number of timesthe peaks drop below the lower reference signal.

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the Figures herein,could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of apparatus and methods in accordance with the presentinvention, as represented in the Figures, is not intended to limit thescope of the invention, as claimed, but is merely representative ofcertain examples of presently contemplated embodiments in accordancewith the invention. The presently described embodiments will be bestunderstood by reference to the drawings, wherein like parts aredesignated by like numerals throughout.

Referring to FIG. 1, in selected embodiments, a fingerprint sensor inaccordance with the invention may include fingerprint sensing area 10 toprovide a surface onto which a user can swipe a fingerprint. A dottedoutline of a finger 11 is shown superimposed over the fingerprintsensing area 10 to provide a general idea of the size and scale of thefingerprint sensing area 10. In certain embodiments, the fingerprintsensing area 10 may include an array 12 of transmitting elements 14,such as a linear array 12 of transmitting elements 14, to assist inscanning a fingerprint as it is swiped across the fingerprint sensingarea 10. The transmitting elements 14 are not drawn to scale and mayinclude several hundred elements 14 arranged across the width of afingerprint. A fingerprint image may be created by scanning successivelines of a finger as it is swiped over the array 12, similar to the waya fax image is captured using line-by-line scanning

In certain embodiments, each transmitting element 14 may successivelyemit a probing signal, one after the other. As will be explained in moredetail hereafter, the probing signal may include a series of probingpulses, such as a series of square waves. A square wave may be usedbecause it is simple waveform that is easy to generate.

In certain embodiments, the probing signal emitted by each transmittingelement 14 may be detected on the receiving end by a receiving element16. In selected embodiments, pairs of receiving elements 16 may be usedto cancel out noise. Like the probing signal, the response signal mayinclude a series of response pulses generated in response to the probingpulses. The magnitude of the response signals may depend on factors suchas whether a finger is present over the fingerprint sensing area 10 and,more particularly, whether a ridge or valley of a fingerprint isimmediately over the transmitting element 14. The magnitude of thesignal received at the receiving element 16 may be directly related tothe RF impedance of a finger ridge or valley placed near the gap betweenthe transmitting element 14 and the receiving element 16.

Instead of using a separate receiving element 16 for each transmittingelement 14, a single receiving element 16 may be used where thetransmitting elements 14 transmit the probing signal at different times.The response signals received by the receiving element 16 may then becorrelated with each transmitting element 14 on the receiving end. Byusing a single receiving element 16, the receiver that is coupled to thereceiving element 16 may be designed to be very high quality and with amuch better dynamic range than would be possible using an array ofmultiple receiving elements.

The design described above differs from many conventional fingerprintsensors, which may employ a single large transmitting element with alarge array of receiving elements and receivers. Nevertheless, many ofthe power management and finger detection features described herein arenot limited to the illustrated transmitter and receiver design. Indeed,the apparatus and methods disclosed herein may be used with fingerprintssensors using a small number of transmitting elements and a relativelylarge number of receiving elements, a large number of transmittingelements and a relatively small number of receiving element, or aroughly equal number of transmitting elements and receiving elements.

In selected embodiments, the fingerprint sensing area 10 (including thetransmitting and receiving elements 14, 16) may be physically decoupledfrom the fingerprint sensing integrated circuit, as will be described inmore detail in FIG. 2. This may enhance the reliability of thefingerprint sensor by reducing the mechanical fragility andsusceptibility to electrostatic discharge that are commonly associatedwith direct contact silicon fingerprint sensors. Positioning the sensingelements off the silicon die also allows the cost of the sensor to bereduced over time by following a traditional die-shrink roadmap. Thisprovides a distinct advantage over direct contact sensors (sensors thatare integrated into the silicon die) which cannot be shrunk to less thanthe width of an industry standard fingerprint. Nevertheless, the powermanagement and finger detection features disclosed herein may beapplicable to fingerprint sensors with sensing elements that are locatedeither on or off the silicon die.

In certain embodiments, the fingerprint sensing area 10 may include oneor more transmitting and receiving elements 18, 20 to “wake up” thefingerprint sensing circuit when a user swipes a finger over thefingerprint sensing area 10. These elements may stay active when otherelements (e.g., elements 14, 16) have been turned off, disabled, or havebeen put into a sleep or hibernation mode. Although only two elements18, 20 are shown, the fingerprint sensing area 10 may include additionaltransmitting and/or receiving elements placed at various locations onthe fingerprint sensing area 10. Like the transmitting and receivingelements 14, 16, the elements 18, 20 may be used to wake up thefingerprint sensing circuit by detecting changes in impedance when afinger is placed or swiped over the fingerprint sensing area 10.

In selected embodiments, certain elements 18, 20 may be dedicated towaking up the fingerprint sensor while other elements 14, 16 may bededicated to scanning fingerprints. In other embodiments, one or more ofthe elements 14, 16, 18, 20 may double as “wake up” elements andfingerprint scanning elements. For example, a small subset of thetransmitting and receiving elements 14, 16 may be kept active even asother elements 14, 16 are turned off or put into sleep mode. When fingeractivity is detected by the small subset, the remainder of thetransmitting and receiving elements may be woken up to begin scanning afingerprint. In selected embodiments, the fingerprint sensing circuitmay be programmable to allow different fingerprint sensing elements 14,16 to be used as “wake up” elements.

Referring to FIG. 2, in selected embodiments, the response signalreceived by the receiving element(s) 16 may be received by an analogfront end 30, where it may be amplified and/or passed through variousfilters to remove noise or other unwanted components from the signal.The amplified and/or filtered analog signal may then be passed to ananalog-to-digital converter (ADC) 32, where it may be converted to asequence of bits. For example, in selected embodiments, the responsesignal may be converted to a string of bytes (or a “line” of data), onebyte for each transmitting element 14 in the array 12. Each byte mayrepresent one of 256 (i.e., 2⁸) possible values depending on themagnitude of the response signal received for each transmitting element14. As mentioned previously, the magnitude of the response signal maydepend on factors such as whether a finger is placed over thefingerprint sensing area 10 and more specifically whether a ridge orvalley of a fingerprint is present over a transmitting element 14.

Digital data output by the ADC 32 may then be stored in a FIFO buffer38. The FIFO 38 may be coupled to a bus 40, which may communicate withvarious components, such as a CPU 42, memory 44, direct memory accesscontroller 46, and the like. The bus 40 may also communicate with one ormore interfaces, such as a USB interface 48, Serial Peripheral Interface(SPI) interface 50, parallel port (PP) interface 52, or the like. TheFIFO 38 may provide a data storage medium to compensate for differencesin timing and transfer rate between the components 30, 32, 38 and thecomponents 42, 44, 46, 48, 50, 52.

In selected embodiments, some or all of the components illustrated inFIG. 2 may be implemented in an integrated circuit 56. This integratedcircuit 56 may communicate with a host system 54 through one or more ofthe interfaces 48, 50, 52. The host system 54, for example, may processthe fingerprint data using various matching algorithms in order toauthenticate a user's fingerprint.

Referring to FIG. 3, as mentioned previously, incorporating afingerprint sensor or other biometric sensor into an electronic devicetypically requires use of additional hardware and software. Thishardware and software increases the power demands already placed onbatteries or other energy sources for an electronic device. Thus, itwould be an improvement in the art to significantly reduce the powerthat is consumed by a fingerprint sensing circuit in accordance with theinvention.

In selected embodiments in accordance with the invention, the integratedcircuit 56 described in FIG. 2 may be divided into various power domainsin an effort to reduce the power that is consumed by the integratedcircuit 56. For example, the integrated circuit 56 may be divided intoan “always on” power domain 60, a “low” power domain 62, and a “core”(or primary) power domain 64. Electrical power supplied to each of thesepower domains may be selectively turned on/off or enabled/disabled toput the integrated circuit 56 into various power modes or levels ofactivity.

As will be explained in more detail hereafter, the power domains 60, 62,64 may, in certain embodiments, operate at different voltages. Forexample, components within the “always on” and “low” power domains 60,62 may operate at 3.3V (the voltage supplied to the integrated circuit56), whereas components within the “core” power domain may operate at1.2V. Level shifters 65 may be placed between the power domains 60, 62,64 operating at different voltages to change the voltage of signalstransmitted therebetween. Although the higher operating voltages of the“always on” and “low” power domains 60, 62 may require use of largercomponents (e.g., transistors, resistors, etc.), which may increasepower consumption, the increased power consumption may be offset byeliminating voltage regulators needed to reduce the operating voltage.That is, by eliminating the voltage regulators for the “always on” and“low” power domains 60, 62, the net power consumption may be reducedeven while using larger, less efficient components (assuming that thenumber of larger components is relatively small).

In certain embodiments, the “always on” power domain 60 may include verylow power control logic 66 which may remain operational even when otherportions of the integrated circuit 56 are turned off or disabled. Incertain embodiments, the “low” power domain 62 may include a“wake-on-event” module 68 which may listen for finger or non-fingeractivity and wake up other parts of the circuit 56 (e.g., corecomponents 70) when such activity is detected. The “wake-on-event”module 68 may include both low power digital components 69 and analogcomponents 71, as will be explained in more detail hereafter. The “core”power domain 64 may include core components 70, which may include coredigital components 72 and core analog components 74. Core digitalcomponents 72, for example, may include the CPU 42, memory 44, DMAC 46,and other digital components. The core components 70 may comprise thelargest portion of the circuit 56 and may provide most of the processingpower for the circuit 56.

In certain embodiments, the very low power control logic 66 may controlthe power supply to each of the power domains 60, 62, 64. For example,the very low power control logic 66 may turn power on and off tocomponents in the low power domain 62. The very low power control logic66 may also control the power supply to voltage regulators 76, 78, whichmay reduce the operating voltage from 3.3V to 1.2V to power the corecomponents 70. These regulators 76, 78, in selected embodiments, mayinclude a core digital and core analog voltage regulator 76, 78, each ofwhich may be turned on and off independently. By controlling powersupplied to the regulators 76, 78, the core analog and core digitalcomponents may be turned on or off as needed.

In certain embodiments, output latches 80 may be provided to latch orgate output values to fixed states when the components within particularpower domains are powered off. That is, when components within aparticular power domain are powered off, the outputs from thesecomponents may “float” or assume uncertain states that may confusedownstream components, which may be unaware that the upstream componentsare powered off. To correct this problem, the outputs may be latched orgated to fixed states to avoid confusion or erratic behavior that mayresult when various power domains or components are powered off.

Referring to FIG. 4, while continuing to refer generally to FIG. 3, theintegrated circuit 56 may operate in one of several modes, or states ofoperation, which may affect the power supplied to the power domains 60,62, 64. For example, the integrated circuit 56 may operate in one of asleep mode 80, a “wake-on-event” mode 82, and an active mode 84.

Sleep mode 80 may refer to the circuit's lowest level of hibernation orinactivity and may provide the lowest power consumption. In this mode80, the “low” power domain 62 and “core” power domain 64 (both theanalog portion 74 and digital portion 72) may be turned off.Furthermore, the 1.2V and 3.3V level shifters 65 may be disabled and theoutputs may be latched or gated to fixed states. In sleep mode 80, thefingerprint sensing elements (i.e., the transmitting and receivingelements 14, 16, 18, 20) may be shut down such that they are neithertransmitting nor receiving. As a result, finger activity over thefingerprint sensing area 10 will go undetected and thus have no effecton the fingerprint sensor.

“Wake-on-event” mode 82 may correspond to a slightly greater level ofactivity and power consumption compared to sleep mode 80, while stillleaving most of the circuit 56 (i.e., the core components 70) turnedoff. Thus, “wake-on-event” mode 82 may be similar to sleep mode 80 inthat it consumes very little power, while providing some additionalfeatures not provided by sleep mode 80. In “wake-on-event” mode 82, the“low” power domain 62 may be turned on while the “core” power domain 64(both the analog portion 74 and digital portion 72) may be turned off.The 1.2V and 3.3V level shifters may be disabled and the outputs may belatched or gated to fixed states. In general, the “wake-on-event” mode82 may differ from sleep mode 80 in that finger activity over thefingerprint sensing area 10 may be detected and used to wake up the restof the circuit 56.

As will be explained in more detail in association with FIG. 9,“wake-on-event” mode 82 may alternate between two sub-modes to reducepower consumption even further. These sub-modes may include (1) a lowerpower “wait for event” sub-mode where the low power analog components 71in the wake-on-event module 68 are disabled and the power domain 62 bfor the switched outputs is turned off; and (2) a higher power “sensing”sub-mode where the low power analog components 71 are enabled and thepower domain 62 b for the switched outputs is turned on.

When operating in the “sensing” sub-mode, a relatively small number offingerprint sensing elements (i.e., the transmitting and receivingelements 14, 16, 18, 20) may be kept active to sense activity over thefingerprint sensing area 10 and wake up the circuit 56 and the remainingfingerprint sensing elements. When operating in the “wait for event”sub-mode, the fingerprint sensing elements may be turned off while the“wake-on-event” module 68 may continue to listen for non-finger activitysuch as General Purpose Input/Output (GPIO) activity, USB activity, SPIactivity, parallel port activity, the expiration of one or more timers,or the like. Non-finger activity, like finger activity, may be used towake up the circuit (e.g., the core components 70) and the remainingfingerprint sensing elements in order to read a fingerprint. In selectedembodiments, the integrated circuit 56 may be programmable with respectto the type of activity that will wake up the core components 70.

Although the lowest power may be achieved when all non-essentialcomponents (including the CPU 42) are turned off, in selectedembodiments the CPU 42 may be kept active in wake-on-event mode 82. Ifdesired, other core components 70, such as the memory 44 or DMAC 46 maybe disabled or turned off completely to conserve power. By keeping theCPU 42 active, the CPU 42 may be available for quick response and/orcalibration. This may allow the CPU 42 to respond more rapidly to eventsby eliminating the need to power up and go through initializationroutines. As will be explained in more detail in association with FIGS.6 through 9, this may also allow faster calibration of the upper andlower references signals and amplifier gains.

In active mode 84, various core components 70, most notably the CPU 42,may be turned on to resume normal, full power operation. This maysignificantly raise power consumption while also significantlyincreasing the functionality of the circuit 56. Unlike the other twomodes 80, 82, the 1.2V and 3.3V level shifters may be enabled and theoutputs may be opened (i.e., not latched or gated to fixed states). Likethe “wake-on-event” mode 82, the active mode 84 may include varioussub-modes. These sub-modes may include (1) a lower power “idle” sub-modewhere the core digital components 72 (e.g., the CPU 42, the memory 44,etc.) are turned on but the core analog components 74 (e.g., the analogfront end 30, ADC 32, etc.) are turned off; and (2) a higher power“scanning” sub-mode where both the core digital components 72 and coreanalog components 74 are turned on. The “scanning” sub-mode may be usedto scan a fingerprint and the “idle” sub-mode may be used during periodswhere scanning is not performed, will imminently be performed, has justended, or the like, such that the analog components 74 are not neededbut the CPU 42 is still turned on or active.

Various events may cause the circuit 56 to transition between each ofthe modes 80, 82, 84. For example, a “wake up” event may cause thecircuit 56 to transition from sleep mode 80 directly to active mode 84.For example, if a fingerprint sensor is implemented in a “flip type”cell phone, opening up the cell phone may expose the fingerprint sensorand may be considered a “wake up” event 86 to wake up the fingerprintsensor and put it into active mode 84. This will cause the fingerprintsensor to begin scanning the fingerprint sensing area 10 for afingerprint. Similarly, closing the cell phone may be considered a “goto sleep” event 88 which may cause the circuit 56 to transition back tosleep mode 80. Such a transition 88 may occur, for example from eitherthe active mode 84 or the wake-on-event mode 82.

On the other hand, if the cell phone is left open (with the fingerprintsensor exposed), but the sensor has not detected finger activity forsome time (t), the circuit 56 may transition 90 from active mode 84 to“wake-on-event” mode 82. This will cause the circuit 56 to transition toa much lower power state while still retaining the ability to listen forfinger or non-finger activity. More specifically, the wake-on-eventmodule 68 may take control of the circuit 56 and shut off power to thecore components 70, including the CPU 42, to eliminate or reduce bothswitching and leakage current in the core components 70.

On the other hand, if the wake-on-event module 68 detects finger ornon-finger activity (i.e., a finger or non-finger event 92), the circuit56 may transition 92 back to active mode 84. More specifically, thewake-on-event module 68 may turn on power to the core components 70 andpass control back to the

CPU 42. By shutting down the primary logic portion (e.g., the corecomponents 70) of the integrated circuit 56 and passing control to asecondary logic portion (e.g., the wake-on-event module 68) duringperiods of inactivity, very low power consumption may be achieved. Infact, an integrated circuit 56 operating in the manner described hereinhas been shown to consume less then 10 μA (usually 5-6 μA) whenoperating in wake-on-event mode 82.

The illustrated modes 80, 82, 84, including the sub-modes discussedherein, and the events 86, 88, 90, 92 used to transition between themodes 80, 82, 84, simply provide one example of various modes and eventsthat may be used with an integrated circuit 56 in accordance with theinvention. Thus, the illustrated example is not intended to be limiting.Indeed, the modes, names of the modes, components and power domainseffected by the modes, and events used to transition between the modes,may be modified, added to, or subtracted from, as needed, withoutdeparting from the principles and characteristics of the invention asdescribed herein.

Referring to FIG. 5, one embodiment of a wake-on-event module 68 isillustrated. In selected embodiments, a wake-on-event module 68 inaccordance with the invention may include a state machine 100 to providethe logic for the wake-on-event module 68. The state machine 100 may becoupled to an ultra low power oscillator 102 to provide a clock signalto the state machine 100. The state machine 100 may control theoperation of the circuit 56 when in wake-on-event mode 82, includingcontrolling when to turn on power to the core components (e.g., the CPU42) and pass control back to the CPU 42. As will be explained in moredetail in FIG. 6, the wake-on-event module 68 may also include an analogdetector 120 to detect finger activity over the fingerprint sensing area10.

In selected embodiments, before the CPU 42 is turned off, the CPU 42 maybe configured to send various types of data (e.g., settings, calibrationvalues, security values, etc.) to the state machine 100 for retrieval ata later time. This is because the CPU 42 and memory 44 may be powereddown and thus may be unable to retain any data when in wake-on-eventmode 82. This data may be saved in one or more persistent registers 104in the wake-on-event module 68 when the CPU 42 is powered down. The CPU42 may retrieve this data from the registers 104 when power is turnedback on in order to start up and initialize correctly.

The wake-on-event module 68 may also communicate with sensor I/O drivers106 that interface with the transmitting and receiving elements 14, 16,18, 20. These sensor I/O drivers 106 may be shared by the wake-on-eventmodule 68 and the CPU 42 and may be programmable to function as eithertransmitters or receivers. A sensor I/O driver 106 may be provided foreach transmitting and receiving element 14, 16, 18, 20 in thefingerprinting sensing area 10. The wake-on-event module 68 and CPU 42may use decoders 108 to select which sensor I/O drivers 106 are utilizedby the wake-on-event module 68 and CPU 42 respectively.

When in wake-on-event mode 82, the wake-on-event module 68 may takecontrol of the sensor I/O drivers 106. Similarly, when in active mode84, the CPU 42 may take control of the sensor I/O drivers 106. Amultiplexer 110, controlled by the state machine 100, may be used toselect whether the state machine 100 or the CPU 42 controls the sensorI/O drivers 106. One or more sensor I/O select registers 112, coupled toa multiplexer 114 (also controlled by the state machine 100) may storedata that selects which sensor I/O drivers 106 are configured totransmit and receive when in wake-on-event mode 82.

In selected embodiments, the CPU 42 may communicate with thewake-on-event module 68 through a number of I/O lines. These I/O linesmay pass through level shifters 65 due to the difference in operatingvoltages between the CPU and wake-on-event module power domains. Forexample, the I/O lines may include “reset,” “control,” “data,” and“sensor I/O control” lines. In selected embodiments, the reset andcontrol I/O lines may communicate with a demultiplexer 116 to selectwhich registers 104, 112 are written to by the CPU 42. The sensor I/Ocontrol line(s) may communicate with the decoders 108 by way of themultiplexer 110.

Referring to FIG. 6, one embodiment of a detector 120 for use in thewake-on-event module 68 is illustrated. As shown, a transmitting element122 (such as an element 14, 16, 18, 20 configured to act as atransmitter) may be configured to emit a probing signal 124 comprising aseries of probing pulses. In this example, the probing signal 124includes a series of square waves. This probing signal 124 may be pickedup by a receiving element 126 (such as an element 14, 16, 18, 20configured to act as a receiver) to generate a response signal.

The response signal may be received by the detector 120 of thewake-on-event module 68. In certain embodiments, the response signal maybe passed through various analog components, such as a high pass filter128 to remove noise, and an amplifier 130. The amplifier 130, and moreparticularly the high pass filter 128, may generate a response signal132 that includes a series of response pulses which may resemble aseries of sharp peaks. This response signal 132 may be input to acomparator 134 a, which may compare the response signal 132 to an upperreference signal, and a comparator 134 b, which may compare the responsesignal 132 to a lower upper reference signal. In selected embodiments,the upper and lower reference signals may be digitally programmed to adesired level using a pair of resistive digital-to-analog converters, orRDACs 136 a, 136 b.

As will be explained in more detail in FIGS. 7 and 8, upon comparing theresponse signal 132 to the upper reference signal, the comparator 134 amay output a logical high value whenever the response pulses exceed theupper reference signal. Similarly, the comparator 134 b may output alogical high value whenever the response pulses exceed the lowerreference signal. These high values may be captured by one or morelatches 138 so they may be counted 140 by the state machine 100 and,depending on the count, appropriate action may be taken by thewake-on-event logic 142 of the state machine 100.

Referring to FIG. 7, a timing diagram is provided to show the operationof the wake-on-event module 68, and more specifically the detector 120.As shown, a transmitting element 122 may emit a probing signal 124comprising a series of square waves. This probing signal 124 may bedetected by a receiving element 126, where it may be passed through anamplifier 130 and a high pass filter 128 to generate a response signal132. As shown, the response signal 132 may comprise a series of responsepulses which may resemble a series of sharp peaks. These sharp peaks maycorrespond to the leading and trailing edges of the square waves 124(the high frequency component of the square waves) after they passthrough the high pass filter 128.

As shown, an upper reference signal 150 a may be established above thepeaks of the response signal 132 and a lower reference signal 150 b maybe established below the peaks of the response signal 132. As explainedin association with FIG. 6, these references signals 150 a, 150 b may begenerated by the RDACs 136 a, 136 b. Because the fingerprint sensor maynot be able to determine whether a finger was already present on thefingerprint sensing area 10 when sensing begins, the upper referencesignal 150 a may be used to detect whether a finger is placed on thefingerprint sensing area 10 after the reference signals 150 a, 150 bwere established. The lower reference signal, on the other hand, may beused to detect whether a finger was already present on the fingerprintsensing area 10 but was removed from the fingerprint sensing area 10after the reference signals 150 a, 150 b were established. Thewake-on-event module 68 may be configured to wake up the circuit 56 foreither of these events.

For example, when a user places a finger over the fingerprint sensingarea 10, the magnitude of the response signal 132 may increasesignificantly due to the reduced impedance between the transmittingelement and the receiving element. This will cause the magnitude of theresponse pulses 152 to increase above the upper reference signal 152.When the response pulses 152 increase above the upper reference signal150 a, the comparator 134 a may output a high logic value, as shown bythe comparator output signal 154 a. Similarly, when the magnitude of theresponse pulses 152 is greater than the lower reference signal 150 b,the comparator 134 b may also output a high logic value, as shown by thecomparator output signal 154 b. In this example, the peaks of theresponse signal 132 always exceed the lower reference signal 150 b andthus each response pulse will generate a pulse on the comparator outputsignal 154 b.

As explained previously, the pulses of the comparator output signals 154a, 154 b may be captured and held by one or more latches 138. Thus, afirst latch output 156 a may generate a high value each time a highvalue is encountered in the comparator output signal 154 a. A secondlatch output 156 b may generate a high value each time a high value isencountered in the comparator output signal 154 b. These latches 138 maybe reset after each pulse so they may be counted by the state machine100 and be ready to capture and hold the next pulse on the comparatoroutput signals 154 a, 154 b. After the number of pulses of the first andsecond latch output signals 156 a, 156 b have been counted, the numbermay be processed by the wake-on-event logic 142 in the state machine 100to take action. Based on the number of pulses that are counted above orbelow the reference signals 150 a, 150 b, the wake-on-event module 68may determine that finger activity has been detected and wake up thecircuit 56.

Referring to FIG. 8, another example of a timing diagram is illustratedto show the operation of the wake-on-event module 68 and the detector120. In this example, upper and lower reference signals 150 a, 150 b areestablished above and below the peaks of the response signal 132.However, after some duration, the peaks 160 of the response signal 132fall below the lower reference signal 150 b. This may occur where afinger was already present on the fingerprint sensing area 10 when thereference signals 150 a, 150 b were established, but was removedthereafter. Accordingly, the output 154 a from the comparator 134 astays low since none of the peaks exceed the upper reference signal 150a. However, the output 154 b from the comparator 134 b stays low onlyafter the peaks drop below the lower reference signal 150 b. When theoutputs 154 a, 154 b do go high, these values may be captured and heldby one or more latches 138, as reflected by the latch output signals 156a, 156 b. These high values may then be counted 140 and processed by thewake-on-event logic 142.

Referring to FIG. 9, in selected embodiments, each transmitting element(i.e., T₁, T₂, T₃, etc.) configured to detect finger activity when inwake-on-event mode 82 may periodically transmit a series of probingpulses, such as a series of sixteen pulses, separated by some amount ofidle time. That is, idle time may be present between the series ofpulses sent to each transmitter. In certain embodiments, thewake-on-event module 68 may include a programmable timer to adjust theamount of idle time between each series of pulses. During the idle time,the wake-on-event module 68 may be put in the lower power “wait forevent” sub-mode described in association with FIG. 4. That is, the lowpower analog components 71 (e.g, the detector 120, the sensor I/Odrivers 106, etc.) may be disabled and the power domain 62 b for theswitched outputs may be turned off.

However, the wake-on-event module 68 may still be configured to detectnon-finger activity, such as GPIO activity, USB activity, SPI activity,parallel port activity, or the like, even during the idle time. Inselected embodiments, the state machine 100 may include a programmabletimer to automatically wake up the core components 70 after a specifiedamount of time has passed, regardless of whether finger or non-fingeractivity was detected. By disabling or turning off the analog components71 during idle time, the power that is consumed by the wake-on-eventmodule 68 during idle time may be significantly reduced.

Conversely, the wake-on-event module 68 may be put in the higher power“sensing” sub-mode when actively transmitting and receiving from thefingerprint sensing area 10. In this sub-mode, the low power analogcomponents 71 may be enabled and the power domain 62 b for the switchedoutputs may be turned on.

Each time the series of probing pulses is transmitted to the fingerprintsensing area 10, the response signal 132 may be compared to the upperand lower references signals 150 a, 150 b, as described in FIGS. 7 and8. A counter 140 in the state machine 100 may count the number of timesthe response is above the upper reference signal 150 a and the number oftimes the response is above the lower reference signal 150 b.

If no event occurs (i.e., a finger is neither placed on nor removed fromthe fingerprint sensing area 10), the counter 140 should count zeroresponses above the upper reference signal 150 a and sixteen responsesabove the lower reference signal 150 b. On the other hand, if a fingeris placed over the sensor, the counter 140 should count sixteenresponses above the upper reference signal 150 a and sixteen responsesabove the lower reference signal 150 b. Similarly, if a finger wasalready placed on the fingerprint sensor when the sensor enterswake-on-event mode 82 but was then removed, the counter 140 should countzero responses above the upper reference signal 150 a and zero responsesabove the lower reference signal 150 b.

These scenarios are the ideal cases where the counter 140 counts eitherzero or sixteen responses for all cases. In practice, the counter 140will likely arrive at a number between zero and sixteen due to theeffect of noise or other variations in the response signal 132. Inselected embodiments, the wake-on-event logic 142 may be programmed totrigger a wakeup for different numbers of response pulses that cross thereference signals 150 a, 150 b. For example, the wake-on-event logic 142may be programmed to trigger a wake-up if ten of the sixteen responsesare above the upper reference signal 150 a. Similarly, the wake-on-eventlogic 142 may be programmed to trigger a wake-up if four of the sixteenresponses are below the lower reference signal 150 b. These numbers maybe adjusted as needed to account for noise and other signalfluctuations.

As shown in FIG. 9, in other embodiments, the wake-on-event module 68may be configured to transmit the probing pulses to the transmittingelements back-to-back, separated by idle time. That is, all thetransmitting elements may be fired in succession, or in “bursts,”followed by idle time between the bursts. Like the previous example, thewake-on-event module 68 may be put in the lower power “wait for event”sub-mode during idle time. Similarly, the wake-on-event module 68 may beput in the higher power “sensing” sub-mode when it is activelytransmitting and receiving from the fingerprint sensing area 10.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described examples areto be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

1. A method for detecting the presence of a finger on a fingerprintsensor, the method comprising: transmitting a probing signal to afingerprint sensing area, the probing signal comprising a series ofprobing pulses; receiving a response signal from the fingerprint sensingarea in response to the probing signal, the response signal comprising aseries of response pulses; establishing an upper reference signal; andmonitoring whether the peaks of the response pulses exceed the upperreference signal.